A polarization element that provides polarized light is known and used in various products and devices such as daily use products (e.g., polarizing sun glasses) as well as optical elements (e.g., polarization filters and polarization films). Also, the polarization element is often used in display devices such as a liquid crystal display. The polarization element is categorized into several groups, depending upon the manner of extracting the polarized light. One of such groups is a wire grid polarization element.
The wire grid polarization element includes a transparent substrate (or board) and a fine stripe-like grid formed on the transparent substrate, and the grid is formed from metal (conductor). The gap or distance between each two adjacent linear parts of the grid is designed to be smaller than the wavelength of the light to be polarized, and therefore the grid can serve as a polarization element (polarizer). Of the linearly polarized light, the polarized light that has an electric field component in the length direction of each linear part is reflected by the grid because the grid (polarization element) is equivalent to the flat metal, whereas the polarized light that has an electric field component in the direction perpendicular to the length direction of each linear part passes through the transparent substrate (polarization element) and exits from the polarization element because the grid is equivalent to only the transparent substrate. Thus, the linearly polarized light that has the electric field component in the direction perpendicular to the length direction of each linear part is only emitted from the polarization element. If the posture of the polarization element is controlled such that the length direction of each linear part of the grid is directed in a desired direction, then it is possible to obtain the polarized light that has the axis of the polarized light (direction of the electric field component) in a desired direction.
For the sake of description in the following passages, the linearly polarized light that has the electric field directed in the length direction of each linear part of the grid is referred to as “s polarized light” and the linearly polarized light that has the electric field directed in the direction perpendicular to the length direction of each linear part of the grid is referred to as “p polarized light.” Generally, the light (wave) that has an electric field in the direction perpendicular to the incident plane (plane perpendicular to the reflection surface and including the incident light and the reflected light) is referred to as “s wave” and the light (wave) that has an electric field in the direction parallel to the incident plane is referred to as “p wave,” but in this specification the terms “s polarized light” and “p polarized light” are used on the premise that the length direction of each linear part is perpendicular to the incident plane.
The basic indices that show the performances and characteristics of such polarization element include an extinction ratio ER and transmissivity (transmittance) TR. The extinction ratio ER is a ratio of the intensity of one polarized light passing through the polarization element to that of another polarized light passing through the polarization element, i.e., the ratio of the intensity of the p polarized light (Ip) to the intensity of the s polarized light (Is) (ER=Ip/Is). The transmissivity TR is a ratio of the energy of the p polarized light exiting from the polarization element to the total energy of the incident s polarized light and p polarized light (Iin) (TR=Ip/Iin). An ideal polarization element has the extinction ratio ER of ∞ (ER=∞) and the transmissivity of 50% (TR=50%).
It should be noted that the polarization element of the present invention is not limited to a grid that is made from the metal (wire). In the following description, therefore, the polarization element is simply referred to as a “grid polarization element.”